1. Field of the Invention
The present invention relates to an apparatus, method, program and storage medium for activity recognition. More specifically, the present invention relates to an activity recognition apparatus, an activity recognition method, a program to cause a computer to perform such a method, a storage medium or memory storing such a program, and a signal coded to cause a computer to perform such a method.
2. Description of Related Art
Human activity classification has been attempted with cameras, microphones (Non-patent Document 5), inertial sensors (Non-patent Document 4), and goniometers. For each sensor or combination of sensors, researchers have also developed custom algorithms that to varying degrees can classify activity. Since one part of human activity classification is obtaining accurate measurements of human motion, the technology used overlaps with methods used for character animation, kinesiology and biomechanics. However, since the inventor of the present application are addressing the problem of how to determine and classify human activities in a mobile and unconstrained context, all except a few sensor modalities are appropriate.
Computer Vision
One set of such constrained methods involves monitoring the subject with static cameras or wearable cameras (Non-patent Document 10). Often optical markers are attached to the subject's body to simplify visual tracking. The subject's movement can be inferred from the movement of the markers through an inverse-kinematic model.
While this method has the potential of providing very rich and complete measurements, it can only be used in the well-lit space in view of the cameras, which precludes its use in mobile applications.
Computer Audition
There also examples where researchers have tried using microphones (both on the body and in the environment) to automatically determine a user's activity (see Non-patent Document 2 and Non-patent Document 9). However, major advantages of motion sensors are lower data rate (which translates to much lower computational requirements) and the ability to measure only the motion of the user. In audio, sounds caused by the user's activities are mixed with an ever-changing background of environmental sounds. Separation of these foreground and background is an unsolved problem which does not arise when using motion sensors.
Absolute Joint Angle Sensing
This set of methods involves attaching sensors directly to the subject's body so as to measure the angles of the subject's joint. Possible sensor's are potentiometers and rotational encoders (which both require an exoskeleton to transmit the changes in a subject's joint angles to the sensor), bend sensors (strips of material that are sensitive to bending and flexing), and inertial sensors (accelerometers and gyros, which essentially measure changes in velocity via Newton's First Law of Motion). In clinical situations, goniometers (equivalent to a tilt sensor) are often used to measure angles with respect to gravity.
Inertial Sensing
Of all these only the inertial sensors and tilt sensors can be used to measurement of a subject's movements without requiring extensive modifications to clothing (for exoskeletons or strips of flex-sensitive material) or requiring special attachments. Furthermore, inertial sensors can be completely enclosed inside another device that user can simply hold or carry in a pocket or bag, thus making these sensors attractive for use in mobile devices or as self-contained devices.
Other relevant works are disclosed, for example, in Non-patent Document 7, Non-patent Document 8, Patent Document 1, Patent Document 2.